1. Field of the Invention
The present invention pertains to the manufacture of composite material parts, such as those made of fiber reinforced plastic, metal, or ceramic. More specifically, the present invention pertains to an apparatus and method of aligning reinforcing fibers in a dielectric alignment fluid by applying an electric field to orient the fibers as desired.
2. Description of the Prior Art
The use of fibers as reinforcement in composite materials is well known in the art. It is also known that optimal reinforcement occurs when the fibers are aligned along critical stress directions in the final part, thus taking full advantage of the fibers' stiffness and strength in reinforcing the part.
Prior art methods of composite part manufacture include manual and automated production techniques, both of which exhibit undesirable limitations. Stated generally, manual techniques are very expensive, and automated techniques are incapable of producing fiber aligned parts in other than a few very specific shapes.
Manual alignment of fibers or continuous filaments involves placing the fibers or filaments in orientations according to the desired strength characteristics of the final part. The fibers or filaments can be placed between layers of the matrix material, or can be in the form of a tape containing the reinforcing fibers as well as the matrix material.
While this method allows reinforcement of the matrix material in virtually any desired pattern, the cost of production is high. Manual placement of fibers is labor intensive, and cannot be easily automated. Thus the cost of the final part is higher than it would be if an automated method was available.
Some solutions to the excessive cost of the manual alignment method include automated techniques such as filament winding, pultrusion, compression and injection molding. Filament winding is a process where fibers are oriented by wrapping around a mandrel, but this technique is generally only useful for producing cylindrical parts. Pultrusion involves pulling continuous fibers through a resin bath and then through a heated die to form its cross sectional shape, but this technique can only produce constant cross section beams and cannot be used to make parts of arbitrary shape. Compression and injection molding can be used with fiber reinforced plastics to produce virtually any shape. No attempt is made to align the fibers into an advantageous orientation in compression molding, and only limited alignment using flow fields is possible in injection molding.
Another solution to the excessive cost of the manual alignment method is a method of aligning fibers by applying a magnetic or electric field, and allowing fibers to fall through this field, thereby aligning the fibers along the field lines while they fall to be collected into a mat below the electrodes. Magnetic field alignment methods are of limited utility, since the fibers need to be ferromagnetic in order to be influenced into alignment by a magnetic field. Since none of the advance fibers in use today are ferromagnetic, they would need to be coated with a ferromagnetic material, substantially increasing the cost of production.
Electric fields are more useful, since they only require that the fiber be electrically conductive, which many of today's advanced fibers are. Thus no additional processing is required prior to alignment of the fibers in an electric field. There are, however, many problems associated with electric field alignment methods. First, the fibers cannot be economically aligned while suspended within the matrix material itself. The matrix material can be in a soft state, allowing for movement of the conductive fibers, but the high viscosity of the typical molten matrix makes alignment slow and difficult. The intensity of the electric field would need to be substantially increased so as to increase the aligning force exerted on the fibers and reduce alignment time, but this can only be done to the point at which the matrix material breaks down and the electric field is shorted out.
In addition, fiber to fiber interactions can prevent proper alignment either within the matrix or within an alignment fluid. As fibers collide with each other, they may physically prevent each other from aligning. Also, prior art electric field alignment techniques are generally directed toward alignment of relatively non-conducting fibers such as wood particles. However, since many of the modern fibers used in production of composite parts are highly conductive, the existing electric field alignment techniques have severe limitations. As the conductive fibers are being aligned, they come into physical contact with each other, forming a chain which locally shorts out the electric field in the vicinity of the chain. They may even gather to form long enough chains to short out the electric field entirely. Reducing the concentration of conductive fibers reduces this effect, but either increases the processing time or reduces the fiber concentration in the vinyl part.
While electric field alignment methods do lend themselves to automated production, they have not provided the flexibility to align fibers into any desired alignment pattern. Such prior art methods are performed between a pair (or multiple pairs) of plate electrodes, and are particularly well suited for aligning fibers into a single linear orientation. The mechanics of these methods do not easily provide facility for either more complex orientations, or for changing alignment patterns while the mat is gathered at the bottom.
Electric field alignment of conductive fibers also suffers from the problem of field distortion near the bottom of the electrodes. As the conductive fibers collect in a mat at the bottom, they form a conductive sheet which tends to short out the electrodes and thus the electric field. One attempted solution to this problem is to provide electrodes that do not reach the bottom, and thus do not contact the fiber mat. As the fibers fall through the electric field formed by parallel plate electrodes, they align according to the resulting parallel electric field lines. However, near the bottom of the electrodes, the field lines are distorted due to the conductive bottom, and as the fibers reach the bottom of the electrodes and are gathered into the mat, they re-align according to the distortions rather than remaining aligned with the desired parallel field lines.
In addition, the viscosity of the alignment fluid affects the processing rates and the overall efficacy of the alignment process, however, prior art electric field alignment techniques do not teach whether there is any utility to be gained from proper selection of alignment fluid viscosity.
Another potential solution to the distortions at the bottom of the electrodes is described in U.S. Pat. No. 4,113,812 to Talbott et al, where a voltage gradient is applied to the mat horizontally. The problem with this method is that it is unsuitable for use with composite material fibers due to their high conductivity. A mat of highly conductive fibers would short out the intended voltage gradient and would generate considerable heat, perhaps sufficient to cause the alignment fluid to boil.
Other limitations in the prior art methods become apparent when a three-dimensional fiber orientation is desired. The technique of allowing fibers to drop through an electric field and collecting them at the bottom is generally incapable of producing a non-planar alignment. While an electric field which is not parallel to the collecting surface could conceivably be created, the fibers would be subjected to a physical force as they contact the collecting surface, which would tend to realign the fibers so as to be parallel to the collecting surface.
Another problem with prior art methods of producing composite material parts is that strong reinforcing fibers tend to be expensive, and previous automated alignment methods do not make efficient use of these fibers. The prior art does not disclose an automated method which the fibers can be advantageously placed in locations of the part where most needed. Rather, the entire part must contain the same concentration of fibers as is needed in the most critical location of the part.
It is evident that there is a continuing need in the prior art for an efficient and automatable method of aligning fibers for the reinforcement of a matrix material in the manufacture of composite parts. There are also continuing needs for a method of aligning highly conductive reinforcing fibers, a method which makes efficient use of expensive reinforcing fibers, a method capable of producing a three-dimensional alignment pattern, and an apparatus capable of effectuating such methods.